In Nz the VMX scene is healthy but fragmented, as the various clubs and individuals that run events are not affiliated to any governing body. So do not follow any one set of rules or MOMs .
This can be a double edged sword , having the freedom to run events and create classes to suit each event is great , any disputes about bike eligibility (not that I can ever remember hearing any here) can be cleared up there and then as the and classes are self policing and the event organisers can have the final say .
On the other side IMHO it would be nice to have a standard set of class rules and definite classes , so when you go to an event else where in the country you know that they will run Pre 75 , pre 78 , Evo etc.
Affiliating with MNZ though would be the death of VMX in NZ as those idiots couldn't run a cake stall at a school fete
Obviously we dont have the numbers you guys do so we tend to combine classes for racing .
Generally the VMX cut off is pre 1986 and I think this is acknowledged by all event organisers , some choose to run twin shock only events (ie classes up to whats known in Aussie as Evo )
In the central north Island a group of blokes run a 5 rd series known as the Kiwi vintage series
http://livewire.gen.nz/livewireblog/This series is now into its 9th season and has proved very popular and successful , it is also regarded by most as our national titles ( some may argue that point ) but it is the biggest and longest running VMX racing series in NZ .
So I will use this series as a guideline to answer as best I can . ( I am not on the organising committee for this series , just a happy punter since rd 1 series 1 ) so apologies for any inaccuracy.
Classes are
Classic 125 , 250 , open .
This was the old pre 75 class ( and is still known as pre 75 witch can be confusing ) it caters for all bikes built pre 1976 and some 76 models 76 Kawasaki Kxs ,76 Honda Crs , 76 Yamaha Yzs and some 76 Euro models .
Evo 125, 250 , open
Bikes that are factory built (ie no modified linkage bikes ) twin shock, drum brake, air cooled built pre 1986 .
Within the Evo classes at series end the first Pre 78 125 , 250 , Open bikes are recognised as the Pre 78 champions .
Linkage 125, 250 , Open
Bikes that are liquid cooled , or have linkage suspension, or disc brakes built pre 1986.
EVO Lites
126cc to 185cc 2 strokes and up to 250cc 4 strokes 1976 to pre 1986 mostly trail and Enduro derived models .
Three rds of racing at each round, races average out at about 10 minutes or 5 laps .